The present invention relates to the field of mobile wireless communication systems and more specifically to methods and apparatus for communication with mobile telephone users (cellular and personal communication systems), mobile wireless data communications, two-way paging and other mobile wireless systems.
In a mobile wireless network, mobile stations (MS) are typically in communications with one base transceiver station (BTS) through up and down radio links. Such ground-based radio links suffer from strong local variations in path loss mainly due to obstructions and line-of-sight attenuation. As MS move from one point to another, their signal path losses go through shadow fading fluctuations that are determined, among other things, by the physical dimension of the obstructions, antenna heights and MS velocity. These variations in path loss must be taken into account in the design of the uplink and downlink radio link resource allocation.
While communicating with a specific home BTS, MS are frequently within the communications range of other BTS. Statistically, due to the distribution of physical obstructions, the shadow fading path loss fluctuations to such other BTS tend to be only weakly correlated with the path loss fluctuations on the link between the MS and home BTS. Frequently, an MS, at any one time and location, has a lower path loss to a different BTS than the serving BTS with which it is communicating.
In high capacity wireless networks, efficient use of spectrum resources is of utmost importance. Dividing network layouts into ever smaller cells and tightening up frequency reuse, is one way to increase spectrum efficiency, but cannot be applied practically everywhere. Prior art studies on frequency reuse in wireless networks using time division multiplexing, such as GSM, show that frequency hopping (FH) can be used to randomize interference. Frequency hopping improves the carrier-to-noise/interference-ratio of radio links and decreases the frame erasure rate (FER). Thus, frequency hopping allows the loading factor in a network to be increased without increasing bandwidth. The benefits of FH become more pronounced as the pool of frequencies used in a region is increased.
For a set of n given frequencies, GSM allows 64×n different hopping sequences that are described by the MAIO (Mobile Allocation Index Offset) and the HSN (Hopping Sequence Number). The MAIO may have as many values as the number of frequencies in the set and the HSN may take 64 different values. Two channels bearing the same HSN but different MAIOs never use the same frequency on the same burst. Two channels using the same frequency list and the same time slot with different HSNs, interfere randomly for 1/nth of the bursts. The sequences are pseudo-random, except for the special case of HSN=0, where the frequencies are used one after the other in order. Pseudo-random sequences have statistical properties similar to random sequences.
Usually, channels in one cell bear the same HSN and different MAIOs since it is desirable to avoid interference between channels in a cell. Since adjacent cells use disjointed frequency sets, they are not interfering. In distant cells using the same frequency set, different HSNs are used in order to gain from interferer diversity. In GSM, the Common Channels do not use frequency hopping. The common channels (FCCH, SCH, BCCH, P AGCH and RACH) use a fixed frequency.
In addition to frequency hopping, fast macrodiversity switching (FMS), as described in the above-identified cross-referenced applications, has been shown to improve carrier-to-noise/interference-ratio in networks where shadow fading, or slow fading, is present by adaptively switching radio channels to the path with the lowest path loss.
Networks suitable for using FMS or FH typically consist of multiple geographically distributed receivers (“collector resources”) and transmitters (“broadcaster resources”) and multiple mobile stations that communicate with collector resources on uplinks and with transmitter resources on downlinks. Frequently, collector and transmitter resources are co-located in base transceiver stations (BTS). Furthermore, multiple BTSs communicate with one or more base controller stations (BSCs) which in turn are connected via communications links with the Public Switched Telephone Network, with the Internet and/or with other facilities.
According to the above-identified cross-referenced applications, mobile users in FMS enabled networks may be communicating on uplink and downlink traffic channels with more than one BTS. In GSM, a traffic channel is defined as having a specific time slot and carrier frequency. Initially, an MS call is setup with one of the multiple BTSs. This BTS is called the home BTS (hBTS) for the call. When during the course of the communications, the radio link path-loss between a particular mobile station, MS, and its hBTS—due to a shadow fading event—becomes higher than the path loss between the particular MS and another BTS belonging to a set of assisting BTSs (aBTS) for the particular MS, the traffic channel is switched from the hBTS to an aBTS. This aBTS then becomes the serving BTS for the MS typically at least for the duration of the shadow fading event.
When an MS is served by an aBTS during FMS operation, the aBTS communicates with the MS on the same radio channel that was established for the hBTS. Such communications may disturb the network frequency plan and may lead to an undesirable change in the interference environment. This change in the interference environment can occur in all FMS enabled networks, including those using FH, and tends to be independent of the frequency reuse plan.
FH has been found to be most beneficial in 1/3 and 1/1 frequency reuse plans. In both these reuse plans, all available traffic channels are used by every BTS in the network. In 1/3 frequency reuse plans, the pool of available frequencies is divided into three frequency sub-pools, and one such frequency sub-pool is assigned to each of the three sectors in every BTS. The one or more of radio resources in each sector share the frequency sub-pool assigned to the sector using FH for all time slots. Cyclical or random FH may be applied, both with the objective to avoid the simultaneous, or overlapping transmission of bursts within a sector using the same frequency. Such simultaneous, or overlapping transmissions of bursts (co-channel bursts) are called collisions (co-channel interference). Furthermore, the frequency hopping sequences in each sector are designed to minimize simultaneous or overlapping transmission of bursts in adjacent frequency channels (adjacent channel interference).
In 1/1 frequency reuse, the entire pool of available traffic radio channels is used by all radio resources in all sectors of all BTSs. To minimize collisions between traffic channels in different BTSs, each BTS is assigned one specific frequency hopping sequence (FHS). All radio resources within the three sectors of a BTS use the same FHS. To avoid collisions between the traffic channels within the BTSs, each radio resource is assigned one specific mobile allocation index off-set (MAIO). These MAIOs are chosen such that the hopping sequences of all radio resources are orthogonal, thereby avoiding collisions between traffic channels in the BTSs. The FHSs assigned to the plurality of BTSs are not orthogonal. Therefore collisions may occur between traffic channels used in different BTSs.
By way of one example, in a network with 1/1 frequency reuse, a mobile station, MSi communicating on a traffic channel, TCH1, with abase transceiver station, BTS1, using hopping sequence, FHSl, and offset, MAIOi, can have collisions with another mobile station, MSj, communicating on traffic channel, TCHj, with base transceiver station, BTSj, using hopping sequence, FHSj, and offset, MAIOj. When such collisions happen, MSi and MSj receive simultaneous or overlapping downlink bursts from BTSi and BTSj at the same frequency. Likewise, BTSi and BTSj receive uplink bursts at the same frequency. Depending on the alignment of the downlink bursts in time, and depending also on the relative signal power levels at the MSi and MSj locations, the mobile stations may not be able to detect one or more bursts correctly. Similarly, the BTSi and BTSj may not be able to detect bursts correctly dependent on alignment and power levels of received bursts.
This problem is exacerbated when FMS and FH are is employed in the same environment. In the above example, when a BTS is a home hBTSi for MSi, BTSj is a home hBTS for MSj, and when during a shadow fading event, BTSj becomes the assistant serving aBTS for MS1, collisions occur between the traffic channel TCHj, used for communications with MSj being served by BTSj, and traffic channel TCHi, used for communications with MSi, also served by BTSj.
While many different wireless networks have been proposed, there is a need for improved wireless networks that achieve the objectives of improved performance and higher density of Mss when both FH and FMS are employed.